TRH-evoked entry of extracellular calcium in GH 4C 1 cells: Possible importance of arachidonic acid metabolites

Abstract

Previous studies have shown that stimulating pituitary GH 4C 1 cells with thyrotropin-releasing hormone (TRH) evoked a biphasic change in cytosolic free Ca 2+ concentration ([Ca 2+] i;): a rapid release of sequestered Ca 2+ due to the production of inositol-1,4,5-trisphosphate, and Ca 2+ entry via both voltage-operated Ca 2+ channels and a presently unknown voltage-independent influx pathway. The aim of the present study was to further evaluate to which extent the TRH-evoked changes in [Ca 2+] i were dependent on entry of extracellular Ca 2+, and which mechanisms participated in regulating this Ca 2+ entry. Pretreatment of the cells with 4-bromophenylacylbromid (an inhibitor of phospholipase A 2), nordihydroguaiaretic acid (an inhibitor of lipoxygenase), and econazole (an inhibitor of both lipoxygenase and cytochrome P-450 enzymes), attenuated the TRH-evoked increase in [Ca 2+] i, suggesting that noncyclooxygenase metabolites of arachidonic acid or cytochrome P-450 metabolites may participate in regulating the TRH-evoked entry of extracellular Ca 2+. Both nordihydroguaiaretic acid and econazole showed a similar inhibition of the Ca 2+ entry, as did SKF 96365, a compound previously shown to inhibit receptor-activated Ca 2+ entry. We also showed that arachidonic acid per se increased [Ca 2+] i, and acidified the cytosol in GH 4C 1 cells in a dose-dependent manner. The effects of arachidonic acid was reversed by addition of BSA to the cell suspension. The calcium entry and the activation of the metabolism of arachidonic acid may thus be important components of the TRH-evoked signal-transduction pathway in GH 4C 1 cells.